فإن أكلَ الصائمُ، أو شربَ، أو جامعَ ناسيًا: لم يُفطر، ولا قضاءَ عليه، ولا كفارةَ.
Translation: If a fasting person eats, drinks, or has intercourse forgetfully, the fast is not broken; neither make-up nor expiation is due.
Nāsiʾan means acting while forgetful of the fast. Under that condition, eating, drinking, or intercourse does not invalidate the fast. The text expressly rules out both make-up and expiation.
The exception concerns genuine forgetfulness, not a conscious choice. When the person remembers fasting, the act must cease immediately and the fast continues.